The present invention is generally related to a printhead for an inkjet printer and more particularly related to a printhead utilizing small dimensions to produce reduced drop weight ink drops.
Inkjet printers operate by expelling a small volume of ink through a plurality of small orifices in an orifice plate held in proximity to a medium upon which printing or marks are to be placed. These orifices are arranged in a fashion in the orifice plate such that the expulsion of drops of ink from a selected number of orifices relative to a particular position of the medium results in the production of a portion of a desired character or image. Controlled repositioning of the orifice plate or the medium followed by another expulsion of ink drops results in the creation of more segments of the desired character or image. Furthermore, inks of various colors may be coupled to individual arrangements of orifices so that selected firing of the orifices can produce a multicolored image by the inkjet printer.
Several mechanisms have been employed to create the force necessary to expel an ink drop from a printhead, among which are thermal, piezoelectric, and electrostatic mechanisms. While the following explanation is made with reference to the thermal ink expulsion mechanism, the present invention may have application for the other ink expulsion mechanisms as well.
Expulsion of the ink drop in a conventional thermal inkjet printer is a result of rapid thermal heating of the ink to a temperature which exceeds the boiling point of the ink solvent to create a vapor phase bubble of ink. Such rapid heating of the ink is generally achieved by passing a pulse of electric current through an ink ejector which is an individually addressable heater resistor, typically for 1 to 3 microseconds, and the heat generated thereby is coupled to a small volume of ink held in an enclosed area associated with the heater resistor and which is generally referred to as a firing chamber. For a printhead, there are a plurality of heater resistors and associated firing chambers--perhaps numbering in the hundreds--each of which can be uniquely addressed and caused to eject ink upon command by the printer. The heater resistors are deposited in a semiconductor substrate and are electrically connected to external circuitry by way of metalization deposited on the semiconductor substrate. Further, the heater resistors and metalization may be protected from chemical attack and mechanical abrasion by one or more layers of passivation. Additional description of basic printhead structure may be found in "The Second-Generation Thermal InkJet Structure" by Ronald Askeland et al. in The Hewlett-Packard Journal, August 1988, pp. 28-31. Thus, one of the walls of each firing chamber consists of the semiconductor substrate (and typically one firing resistor). Another of the walls of the firing chamber, disposed opposite the semiconductor substrate in one common implementation, is formed by the orifice plate. Generally, each of the orifices in this orifice plate is arranged in relation to a heater resistor in a manner which enables ink to be expelled from the orifice. As the ink vapor bubble nucleates at the heater resistor and expands, it displaces a volume of ink which forces an equivalent volume of ink out of the orifice for deposition on the medium. The bubble then collapses and the displaced volume of ink is replenished from a larger ink reservoir by way of an ink feed channel in one of the walls of the firing chamber.
As users of inkjet printers have begun to desire finer detail in the printed output from a printer--especially in color output--the technology has been pushed into smaller drops of ink to achieve the finer detail. Smaller ink drops means lowered drop weight and lowered drop volume. Production of such low drop weight ink drops requires smaller structures in the printhead. Thus, smaller firing chambers (containing a smaller volume of ink), smaller firing resistors, and smaller orifice bore diameters are required.
It is axiomatic in thermal inkjet printer printheads that the orifice plate thickness be no less than approximately 45 .mu.m thick. Orifice plates thinner than 45 .mu.m suffer the serious disadvantage of being too flimsy to handle and likely to break apart in a production environment or become distorted by heat processing of the printhead. Orifice plates are conventionally manufactured by electroforming nickel on a mandrel and subsequently plated with a protective metal layer on the nickel. Conventional wafer handling production equipment cannot maneuver the thin orifice plate for processing in a manufacturing environment. Furthermore, since a multiplicity of orifice plates are produced as one electroform, singulating each orifice plate from the others on the nickel electroform becomes virtually impossible with production equipment when the metal orifice plate is less than 45 .mu.m thick. Even if the production difficulties with thin, conventionally produced, orifice plates were resolved, the thin orifice plates are too prone to distortion due to stresses when the thin orifice plate is positioned and secured on the barrier layer of the printhead.
Conventionally, an orifice plate for a thermal inkjet printer printhead is formed from a sheet of metal which is perforated with a plurality of small holes leading from one side of the metal sheet to the other. There has also been increased use of a polymer sheet through which holes have been ablated as an orifice plate. In the metal orifice plate example, the process of manufacture has been delineated in the literature. See, for example, Gary L. Siewell et al., "The Thinkjet Orifice Plate: a Part With Many Functions", Hewlett-Packard Journal, May 1985, pp. 33-37; Ronald A. Askeland et al., "The Second-Generation Thermal InkJet Structure", Hewlett-Packard Journal, August 1988, pp.28-31; and the aforementioned U.S. Pat. No. 5,167,776, "Thermal InkJet Printhead Orifice Plate and Method of Manufacture".
Since the reduced size printhead firing chamber and orifice bore diameter generate problems with conventional orifice plates such as overheating due to the large heater resistor necessitated by the thick orifice plate and increased susceptibility to particulate contamination in the orifice bore, it is desirable to reduce the thickness of the orifice plate. Since the orifice plate is best manufactured and used with thickness dimensions greater than 45 .mu.m, it is desirable to produce printheads with orifice plates of this thickness or greater. This quandary needs to be solved to obtain low drop weight ink drops.